1. Field of the Invention
The present invention relates to an alignment apparatus using a coherent light source such as laser. More particularly, the present invention relates to an automatic alignment apparatus suitably used in manufacturing integrated circuit elements and the like to carry out an alignment of a mask and a wafer when a circuit pattern is printed on the wafer through the mask.
2. Description of the Prior Art
According to a known automatic alignment method, two alignment marks are usually provided on each of the wafer and the mask so that a predetermined positional relation may be obtained when the mask is laid on the wafer. When the mask is laid on the wafer, the relative positional deviation of these alignment marks from the predetermined position is detected and, depending on the detected deviation, a servo mechanism is controlled to reduce the deviation. The servo mechanism thus controlled moves the mask or the wafer so as to bring them into the predetermined positional relation.
As will be seen from the above, in the known alignment method it is impossible to increase the accuracy of automatic alignment over the accuracy of detection of the position of the alignment marks, since the former is limited by the latter.
In the method for detecting such alignment marks hitherto used, alignment marks overlapping each other are magnified through an optical system and the image in the magnified image plane is converted into an electrical signal by use of a combination of an image detector, a scanning slit and a detecting element. The electrical signal is further processed to obtain the relative positional deviation between the mask and wafer (amount of error). In carrying out this known mark detecting method, the view field of an optical microscope is so illuminated as to have uniform intensity of light over the whole view field. However, it is known that only a small portion of the total quantity of light reflected can be used to actually detect the marks. Therefore, when the contrast of a pattern on the wafer is lowered in the manufacturing process of the integrated circuit elements, the signal-to-noise (S/N) ratio of the photoelectric signal produced based on the light received by the detecting element is markedly reduced and thereby the accuracy of detection of mark position is bound to be deteriorated.
To overcome the above described drawback of the conventional mark detecting method, there has been already proposed an automatic alignment apparatus employing a laser spot scanning system, an embodiment of which is disclosed, for example, in Japanese Patent Application laid open specification No. 12577/1977. According to the newly proposed detecting system, the surfaces of the mask and wafer are scanned by a laser spot of high power intensity and the light scattered by the alignment marks is detected. Therefore, a larger quantity of light can be effectively used for detection of the alignment marks and the accuracy of detection of mark position is improved accordingly.
In the above mentioned patent publication, there is shown and described such an embodiment in which a laser spot scans the mask and the wafer linearly at a uniform velocity to detect the linear alignment marks provided thereon. This embodiment has an advantage in that the scanning laser spot has a broader scanning range compared with an oscillating spot system and therefore it functions properly even when the alignment marks are somewhat apart from the center of the laser spot scanning. However, it has a particular difficulty owing to the fact that the laser spot scans the marks overlapping each other only unidimensionally. In case that the overlapped marks deviate from each other two-dimensionally, a modification is required to measure the amount of error. Namely, it is required that an alignment mark be composed of line segments which are neither parallel nor normal to the scanning direction of the spot and that time intervals of the photoelectric pulses obtained in a time series according to the mark lines be measured and computed so as to obtain the amount of relative positional deviation in two dimensions. Such a scanning method makes the mark position detecting apparatus complicated and increases the possibility of measurement error. For example, in detecting the deviation of position in a coordinate system with x and y axes, there may occur a mutual interference between the x-component and the y-component, so that the amount of deviation in the x direction then detected may include a small portion of deviation in the y direction and vice versa.